1. Field of the Invention
The invention in general relates to an improved lightning hardened cable, and more particularly to a cable which may be used as a tether in an aerostat system.
2. Background Information
The use of lighter than air vehicles as high altitude platforms for electronic euqipment has been known for many years. By suspending electronic payloads from a stabilized platform beneath an aerodynamically shaped balloon, called an aerostat, communication and/or surveillance over a wide area may be accommodated.
Depending upon the mission, the aerostat may be positioned at an altitude of many thousands of meters and securely connected to a ground based mooring system by means of a high strength lightweight tether.
A typical lightweight tether includes a non-metallic central strength member such as may be constituted by a plurality of layers of aromatic amide organic fibers surrounded by metallic braiding over which is formed a relatively flexible outer protective jacket. If the electronic payload is provided with power from a ground station, the tether may also include a central core of electrical conductors as well as signal conducting optical fibers.
The aerostat may be used in areas subject to lightning strikes and, accordingly, the aerostat is provided with a lightning shield which is electrically tied to the tether braid which forms a path for lightning induced current.
Such arrangement has not been entirely satisfactory since significant damage occurs to the tether when the lightning current transfers to the grounded mooring system. Basically, the surge current arcs through the outer jacket to the metallic portion of the mooring system and causes damage to the tether jacket and braid. In addition, for those tethers which have central power conductors, induced voltage in these conductors may exceed the dielectric strength of the non-metallic central strength member thereby causing a penetration and consequent damage.
Tether damage always requires the removal of the damaged portions from service. This represents a costly process since the tether cannot be spliced in any manner to lengthen it after its original manufacture, and accordingly, the removal of the damaged portion necessitates not only the removal of the damaged portion itself but the removal of several hundred to several thousand feet of tether from the damaged portion to the end of the tether. A tether therefore can take only so many lightning strikes before it has to be completely replaced. Additionally, the extensive downtime required for this process is undesirable since it reduces the operational availability of the system.
The present invention provides for a tether which is lighting hardened and does not require the costly section removal procedure employed with present day tethers.